Summers' Lab Research Interests - Bacterial physiology and genetics

    The research in my laboratory focuses on the problem of how bacteria regulate adaptive alterations of their cell morphology and physiology in response to environmental changes.  I utilize the cyanobacterium  Nostoc punctiforme for these studies. 

    As illustrated in the left-hand panels of the figure below, the filaments cyanobacterium Nostoc punctiforme can differentiate from normal vegetative cells (V) into nitrogen fixing heterocysts (H) resistant akinetes (A) or motile hormogonia (Horm) that move by gliding motility.   Our research focuses on identification of genes involved in cellular differentiation of akinetes.  Akinetes are resting cells capable of surviving long periods of desiccation and cold that differentiate from normal vegetative cells due to low light or phosphate starvation.  For an overview of the differentiation potential in N. punctiforme, please see our recent review.

    We have developed plasmid vectors to report gene transcription and have tested them for cell-type specific expression using known promoters.  For instance in the figure below, the cells in panels A and B contain a green fluorescent protein (GFP) reporter fused to the promoter of hetR, known to be expressed in heterocysts.  The right-hand pannel is a view using epifluorescence microscopy to excite the cells containing GFP.  Similarly, the vegetative cell promoter to psaC (panels C and D) and akinete marker gene promoter avaK (panels E and F) are shown.  If the plasmid reporter vector does not contain a promoter, no expression of GFP occurs (panels G and H).  Please see the publication; Aguilar, Yuksek and Summers, 2004.  J. Microbiol. Meth. 59:181 for details.

Nostoc cells

    We have developed a model system for the study of akinetes in this organism using a metabolic mutant lacking glucose-6-phosphate dehydrogenase (encoded by the zwf gene).  The cells of this mutant synchronously turn into akinetes after several days in the dark if given a carbon source such as fructose.  Please see Argueta and Summers, 2005, Arch. Microbiol. 183:338-346 for more information. 

    Using differential display on RNA harvested from wild-type (grows in the dark as a heterotroph) and mutant (differentiates into akinetes) allowed us to identify genes more highly expressed in akinetes after a dark shift.  Promoters from these genes have been tested using the reporter plasmids shown above and have been confirmed to be more highly expressed in developing akinetes, as shown below.  Please see Argueta, C., K. Yuksek, R. Patel, and M.L Summers. 2006 for more information.
Akinete gene expression

The next step in analysis is to mutate the identified gene and see if functional akinetes form in the mutant strain. 

Recently initiated work:  Our current research is focused on identification and characterization of akinete-related genes identified using a whole genome DNA microarray made at the UC Davis. In this work RNA from the model system as described above was hybridized to the DNA microarray.  Genes that were on or off in the mutant after 3 days incubation in the dark (as compared to the wild-type strain under similar conditions where no akinete differentiation occured) were identified.  Many students are now involved in confirmation of cell-type gene expression and in the construction of mutants in the identified genes.  Students interested in cellular development and molecular genetics are usually involved in this type of lab project. 

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